<?xml version="1.0" encoding="utf-8"?>
 <group xmlns="http://relaxng.org/ns/structure/1.0">
   <attribute name="id">
     <ref name="id-content"/>
   </attribute>
   <attribute name="available">
     <ref name="available-content"/>
   </attribute>
   <element name="isbn">
     <ref name="isbn-content"/>
   </element>
   <element name="title">
     <ref name="title-content"/>
   </element>
   <zeroOrMore>
     <element name="author">
       <ref name="author-content"/>
     </element>
   </zeroOrMore>
   <zeroOrMore>
     <element name="character">
       <ref name="character-content"/>
     </element>
   </zeroOrMore>
 </group>
